cmd/compile: remove redundant SEQZ/SNEZ on riscv64
In particular, (SEQZ (SNEZ x)) can arise from (Not (IsNonNil x)).
Change-Id: Ie249cd1934d71087e0f774cf8f6c937ceeed7ad5
Reviewed-on: https://go-review.googlesource.com/c/go/+/428215
Run-TryBot: Joel Sing <joel@sing.id.au> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Wayne Zuo <wdvxdr@golangcn.org>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com>